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Why does a program defined by an ado-file sometimes not work when given a string comparison?

Title   Ado-files and string comparisons
Author Nicholas J. Cox, Durham University, UK

Sometimes programs defined by ado-files produce puzzling output when called with an if condition followed by a string comparison.

This can usually be traced to a bug (or limitation) affecting commands defined by programs with version less than 6.0. Such programs have an early line something like

        version 5.0

which may be seen by looking at the code in a text editor or by typing the file

        type location/whatever.ado

This version statement (see [P] version or the online help for version) means that what follows is interpreted with Stata behaving as Stata 5.0 (even if you have Stata 6.0 or later).

Under version 5, and before that, quotes (" ") were stripped by Stata from arguments to programs defined by ado-files. Suppose that whatever.ado defines command whatever. What whatever sees when you type

        whatever mpg weight if substr(make,1,4) == "Chev"

are the arguments

        mpg weight if substr(make,1,4) == Chev

Generally, there are two possibilities:

  • You have no variable Chev. If that is the case, you will get an error message, as Stata cannot find the variable you specified.
  • You have such a variable, but in that case, it is very likely that you would get results other than you intended.

There is a work-around. Create a variable that will have the correct effect.

        gen byte Chev = substr(make,1,4) == "Chev"
        whatever mpg weight if Chev

That is, you keep the string comparison out of sight of whatever and create an indicator variable that is 1 or 0 depending on whether the relation is true or false. Stata commands that are defined in the executable do not suffer from this problem: generate is one such command.

This is a rare example of users being affected by whether a command is defined by an ado-file or as part of the executable. Typically, that is a consideration for programmers only (apart from performance issues, sometimes).

More ambitiously, you could try editing the code in whatever.ado, but even expert Stata programmers might be very wary of doing that because there might be side effects.

This bug was fixed in Stata 6.0, and programs defined in ado-files under version 6.0 or later do not suffer from the same problem.